1. Field of Invention
The invention relates to a laminated piezoelectric element for use as a drive device in various apparatus, such as an ink-jet print head.
2. Description of Related Art
A piezoelectric element is used as a drive device (piezoelectric actuator) for various apparatus due to a characteristic of converting electric energy into mechanical displacement (deformation) by piezoelectric effects. In order to increase an amount of displacement by the deformation of a piezoelectric element, the piezoelectric element includes laminated piezoelectric sheets formed of ceramic material, such as lead zirconate titanate (PZT). The piezoelectric sheet has individual electrodes or common electrodes formed on a surface (larger face) thereof with electrical conductive material of, for example, paste. A plurality of the piezoelectric sheets having the individual electrodes formed thereon and a plurality of piezoelectric sheets having the common electrodes formed thereon are alternately stacked on top of each other in layers.
In the thus laminated piezoelectric element, to electrically connect between the individual electrodes or between the common electrodes adjacent in a sheet laminated direction, through holes are provided to which electrically conductive material is applied. FIGS. 17 through 19 show an example of a known laminated piezoelectric element (piezoelectric actuator). FIG. 17 is an exploded view of a piezoelectric actuator 100. FIG. 18 is a sectional view of the actuator 100, taken along line 1005xe2x80x941005 of FIG. 17. FIG. 19 is an explanatory view of the actuator 100 deformed by firing.
The conventional piezoelectric actuator 100 includes a piezoelectric sheet 103a, 103c, 103e, 103g having individual electrodes 101 formed thereon, and a piezoelectric sheet 103b, 103d, 103f, 103h having common electrodes 102 formed thereon, that are alternately laminated, and an insulating sheet 106 disposed on the top. The individual electrodes 101 are formed on the piezoelectric sheet 103a (103c, 103e, 103g), which is odd-numbered when counted from the lower side of the actuator 100. The individual electrodes 101 are provided so as to laterally extend along the shorter side of the piezoelectric sheet 103a (103c, 103e, 103g) toward a central portion thereof. A row of the individual electrodes 101 is provided parallel to the longitudinal direction of the sheet 103a (103c, 103e, 103g) along each longer side of the sheet 103a (103c, 103e, 103g). The common electrodes 102 are formed on the piezoelectric sheet 103b (103d, 103f, 103h), which is even-numbered when counted from the lower side of the actuator 100. The common electrode 102 is provided in a substantially central portion of the piezoelectric sheet 103b (103d, 103f, 103h). The common electrode 102 extends along the longitudinal direction of the piezoelectric sheet 103b (103d, 103f, 103h), forming a substantially rectangular shape.
In the piezoelectric sheets 103a through 103h, piezoelectric active portions 107 that are deformed by the piezoelectric effects are provided at positions sandwiched between the individual electrodes 101 and the common electrodes 102. Extending portions 102a are integrally formed with the common electrode 102 and extend laterally so as to cover a substantially entire length of each shorter side end of the even-numbered piezoelectric sheet 103b (103d, 103f, 103h). Individual dummy electrodes 104 are formed so as to correspond to the individual electrodes 101 (in the vertically same positions), on the surfaces of the even-numbered piezoelectric sheet 103b (103d, 103f, 103h) other than the piezoelectric active portions 107.
Dummy common electrodes 105 are formed on each of the odd-numbered piezoelectric sheet 103a (103c, 103e, 103g) at positions corresponding to the extending portions 102a (in the vertically same positions). The insulating sheet 106 has surface electrodes 108 associated with the individual electrodes 101 and surface electrodes 109 associated with the common electrodes 102, along the longer sides of the sheet 106. Except for the lowermost piezoelectric sheet 103a, through holes 110 are formed on the piezoelectric sheet 103b through 103h and the insulating sheet 106, so as to communicate the surface electrodes 108 with the corresponding the individual electrodes 101 and individual dummy electrodes 104. Similarly, through holes 111 are formed on the piezoelectric sheet 103b through 103h and the insulating sheet 106, so as to communicate at least one surface electrode 109 with the corresponding extending portion 102a. 
The individual electrodes 101 formed on the photoelectric sheets 103a, 103c, 103e, 103g and the associated surface electrodes 108 are electrically interconnected through electrically conductive material applied to the through holes 110. Similarly, the common electrodes 102 formed on the piezoelectric sheet 103b, 103d, 103f, 103h and the associated surface electrodes 109 are electrically interconnected through electrically conductive material applied to the through holes 111. The through holes 110, 111 are provided in a line parallel to an aligning direction of the individual electrodes 101 along the longitudinal direction of the piezoelectric sheet 103b through 103g and the insulating sheet 106, as shown in FIG. 17. The through holes 110, 111 are not formed on the lowermost piezoelectric sheet 103a, to prevent electricity from being conducted to a driven member (e.g., a cavity plate in an ink-jet head) to which the piezoelectric actuator 100 is fixedly attached.
Another known piezoelectric actuator includes an insulating sheet disposed on a larger surface of the piezoelectric sheet laminate. The insulating sheet includes surface electrodes connected to a flexible printed cable to externally and selectively drive the piezoelectric actuator by applying a voltage. The surface electrodes are formed on the insulating sheet so as to be associated with individual electrodes or the common electrodes. Conventionally, the surface electrodes are formed mainly with the following three methods.
As a first method to form the surface electrodes on the insulating sheet, the common electrodes and the individual electrodes are formed on the surfaces of the piezoelectric sheets. A common electrode or individual electrode is extended so as to be exposed on a side face of the piezoelectric sheets. A plurality of the piezoelectric sheets are laminated with the insulating sheet (that has not yet had a surface electrode) placed on the top. Such laminate of the piezoelectric sheets and the insulating sheet is sintered or fired at a high temperature (e.g., approximately 1100xc2x0 C.). Thereafter, electrically conductive Agxe2x80x94Pd (silver-palladium)-based paste is applied to a side end face of the laminate such that side electrodes are formed to connect between the common electrodes or between the individual electrodes in the sheet laminated direction. Then, the surface electrodes are formed on a surface (larger face) of the insulating sheet, so as to be electrically connected to the side surfaces, by applying the same electrically conductive material (paste) as that used for the side electrodes. The surface electrodes are baked at a relatively low temperature (e.g., approximately 600xc2x0 C.).
As a second method, the common electrodes and the individual electrodes are formed on the piezoelectric sheets and the insulating sheet. The through holes are formed on the piezoelectric sheets and the insulating sheet such that the adjacent individual electrodes or the common electrodes in the sheet laminated direction are connected to each other. The same electrically conductive material (paste) as that used for the common electrodes and the individual electrodes is applied to the through holes. Thereafter, the piezoelectric sheets and the insulating sheet are laminated and fired at a high temperatures as described above. Then, the surface electrode is formed on a surface (larger face) of the insulating sheet for each of the through holes by applying electrically conductive Agxe2x80x94Pd (silver-palladium)-based paste, and baking at a lower temperature.
As a third method, the common electrodes and the individual electrodes are formed on the piezoelectric sheets and the insulating sheet. The through holes are formed on the piezoelectric sheets and the insulating sheet such that the adjacent individual electrodes or the common electrodes in the sheet laminated direction are connected to each other. The same electrically conductive material (paste) as that used for the common electrodes and the individual electrodes is applied to the through holes. Tabs, as surface electrodes, are formed on a larger face of the insulating sheet with the same electrically conductive material as that used for the through holes, so that the tabs and the thorough holes are electrically connected. The piezoelectric sheets and the insulating sheet having the tabs formed on the insulating sheet are laminated and fired at a high temperature.
When the piezoelectric sheet 103a through 103h and the insulating sheet 106 are laminated as shown in FIG. 18, the through holes 110, 111 are aligned vertically so as to communicate in the sheet laminated direction, and along the longer side ends of the piezoelectric sheet 103b-103h and the insulating sheet 106. Accordingly, the piezoelectric actuator 100 has continuous low-strength areas near each longer side end thereof along the longitudinal direction.
In addition, the vertically aligned through holes 110, 111 that create low-strength areas, are substantially cylindrical in shape with a bottom of the lowermost piezoelectric sheet 103a. Therefore, the laminate formed of the piezoelectric sheets 103a-103h and the insulating sheet 106 (that is, the piezoelectric actuator 100) shrinks during firing, resulting in deformation, as shown in FIG. 19, so as to close the openings of the through holes 110, 111, when viewed from the lateral direction of the piezoelectric actuator 100. When the piezoelectric actuator 100 having a deformation, such as a curve or a warpage, is used as a drive device for an ink-jet print head, such deformation creates a gap when the piezoelectric actuator 100 is fixed to a surface of a cavity plate by adhesive, leading to ink leakage from the gap.
In the above-described first and second methods to form the surface electrodes, the surface electrodes baked at a lower temperature have lower strength of bonding to the surface of the insulating sheet. Therefore, when the piezoelectric actuator and the flexible printed cable are connected by soldering through the surface electrodes, the surface electrodes are peeled off the insulating sheet, resulting in an unstable electrical connection between the piezoelectric actuator and the flexible printed cable. Even when glass frit is mixed into the electrically conductive paste to improve the bonding strength of the surface electrodes, the degree of the bonding strength improvements is limited.
If the surface electrodes are formed by the above-described third method, the tabs, as the surface electrodes, that are fired at a high temperature shrink. Further, the surface of the tab is oxidized by the heat applied at a high temperature during firing. Therefore, it is difficult to solder the surface electrodes of the piezoelectric actuator and electrodes of the flexible printed cable with the sufficient strength.
One aspect of the invention is to manufacture a reliable piezoelectric actuator that prevents an electrode of the actuator from peeling off or prevents the actuator from warping. Another aspect of the invention is to provide an ink-jet print head including such a reliable piezoelectric actuator.
A laminated piezoelectric element of the invention may include a plurality of sheet members that include at least a plurality of piezoelectric sheets that form a laminate by stacking the plurality of the sheet members, electrode patterns that have at least first electrode patterns including a plurality of individual electrodes on each one of the first electrode patterns formed between the sheet members, and through holes that pierce through at least one of the sheet members which are internal layers of the laminate at least corresponding to the individual electrodes. The through holes electrically connect at least between the first electrode patterns by each of the individual electrodes adjacent in a lamination direction of the sheet members with an electrically conductive material applied to the through holes. The through holes are provided so as to prevent the through holes adjacent in a direction parallel to an alignment of the individual electrodes in the first electrode pattern from aligning along the direction parallel to the alignment of the individual electrodes in the at least one of the sheet members.
A method for manufacturing a laminated piezoelectric element that has a laminate formed by stacking a plurality of sheet members including at least a plurality of piezoelectric sheets, and that has electrode patterns with at least first electrode patterns, including a plurality of individual electrodes on each one of the first electrode patterns formed between the sheet members. The method for manufacturing may include steps of preparing a base sheet, whose size covers a plurality of the sheet members arranged in a matrix, boring though holes in the base sheet, at least at positions where the individual electrodes are due to be provided on each of the sheet members, in such a manner that the through holes are prevented from aligning along a direction parallel to an alignment of the individual electrodes, forming an electrode layer to be the electrode patterns on a surface of the base sheet, using an electrically conductive material and applying the electrically conductive material to the through holes, laminating a plurality of the base sheets and an insulating sheet such that the insulating sheet is placed uppermost, sintering the laminate formed by the base sheets and the insulating sheet, and cutting the laminate that is sintered, into a predetermined size to produce laminated piezoelectric elements.
In the laminated piezoelectric element and the method for manufacturing the laminated piezoelectric element according to the invention, the through holes may be provided in each of the sheet members so as to prevent the through holes adjacent in a direction parallel to an alignment of internal electrodes, such as preventing the individual electrodes from aligning along the direction parallel to the alignment of the internal electrodes. As the sheet members of the piezoelectric sheets having the through holes arranged as described above are laminated, the through holes may be disposed in a staggered configuration when viewed from the top of the laminated sheets. In this case, stresses in the laminated piezoelectric element caused by the shrinkage during firing may be dispersed. Therefore, the amount of deformation after sintering such that the openings of the through holes, which have a lower strength, are closed when viewed from a direction orthogonal to the direction parallel to the alignment of the internal electrodes, may be reduced.
Accordingly, when the laminated piezoelectric element is fixedly attached to a surface of a cavity plate in order to use the laminated piezoelectric element for an inkjet print head as a drive device, creation of the gap (space) between adhesive surfaces of the laminated piezoelectric element and the cavity plate may be prevented. Therefore, problems such as ink leakage may be prevented after the piezoelectric element and the cavity plate, bonded together, are assembled into a product of an ink-jet print head.
A method for manufacturing a laminated piezoelectric element that has a laminate formed by stacking a plurality of sheet members, including at least a plurality of piezoelectric sheets, and that has electrode patterns with at least first electrode patterns, including a plurality of individual electrodes, on each one of the first electrode patterns formed between the sheet members, may include steps of, preparing a base sheet and a insulating sheet whose size covers a plurality of the sheet members arranged in a matrix, forming an electrode layer to be the electrode patterns on a surface of a base sheet using an electrically conductive material, creating an electrode layer to be a pattern of tabs to be associated with the electrode pattern on the insulating sheet using an electrically conductive material, laminating a plurality of the base sheets and the insulating sheet such that the insulating sheet is placed uppermost in such a manner that the pattern of the tabs faces outwardly, sintering a laminate formed by stacking the plurality of the base sheets and the insulating sheet, cutting the laminate that is sintered, into a predetermined size according to the laminated piezoelectric element, forming surface electrodes of a electrically conductive material at least on the tabs to make connection at least between the individual electrodes adjacent in the lamination direction, and baking the surface electrodes.
According to the method of manufacturing the invention, the surface electrodes may be formed on the sheet member (base sheet), through the tabs formed of electrically conductive material. The tabs may be securely bonded to the sheet member at a high temperature while the laminate is sintered. In addition, the tabs and the surface electrodes may be both formed of electrically conductive material, so that the tabs and the surface electrodes may be securely bonded to each other even when the surface electrodes are baked at a low temperature. When the surface electrodes are baked at a low temperature, the surface electrodes will be less subjected to oxidation. Therefore, sufficient bonding strength may be ensured between the surface electrodes and the external connecting device, such as a flexible printed cable.